Many different applications exist in which it is desired to monitor whether an object or a person enters a specific region or leaves the specific region. Thus for example entry of an object or a person into a region or leaving this region can entail an economic or even a safety risk.
In some cases, it appears to be advantageous to mark a limit of a specific region, for example by means of an electrical field or even by a magnetic field. Thus there are systems on the market, for example for monitoring the theft of shopping trolleys in supermarkets, which lock a wheel of the shopping trolley when an area of ground is crossed (e.g. when leaving the car park via an exit) and hence prevent a thief who is trying to steal the shopping trolley from moving further.
Some of these systems comprise a cable which is buried in the ground as an antenna and also a receiver in the wheel (e.g. of the shopping trolley). The cable buried in the ground can be laid for example around the complete area or region to be monitored. Furthermore, the cable buried in the ground is generally connected to a transmitter which produces a modulated (or unmodulated) signal at a specific frequency (e.g. 8 kHz). This signal is emitted by the cable, as a result of which an electromagnetic field is produced around the cable. If for example the shopping trolley approaches the buried line or the buried cable, then the receiver (which is situated for example in the wheel of the shopping trolley) detects the signal (emitted by the cable) from a specific proximity and triggers locking of the wheel. For deactivation of the locking of the wheel, for example a second signal is transmitted either (A) by a second line or (B) by a mobile device. If the receiver receives the second signal, the locking of the wheel can be released again.
If the second signal is emitted by a second line, then it is necessary in many cases to bury a second line in the ground. If the second signal, in contrast, is emitted by a mobile device, then usually an additional operator is required who goes towards the shopping trolley with the device.
In U.S. Pat. No. 5,194,844 a method is described with which activation and deactivation of the locking mechanism can be achieved via only one signal or one buried line. In the case of this method, an asymmetrical (rectangular) signal with a pulse-duty ratio (duty cycle) of less than 40% is transmitted. The signal is received via a receiver coil and evaluated. Because of the asymmetry of the signal, a constant component is produced, on the basis of which a receiver can decide whether it is situated within or outside the monitored region. However, the method requires a special signal shape and therefore cannot be applied in the systems found on the market which use a modulated or unmodulated symmetrical (sine) carrier signal.
A large number of further systems are known furthermore from prior art in order to produce theft protection in shopping trolleys or to enable position determination.
Thus WO 99/03720, U.S. Pat. No. 6,362,728, U.S. Pat. No. 6,945,366, U.S. Pat. No. 6,353,388, U.S. Pat. No. 6,127,927 and U.S. Pat. No. 6,037,869 respectively describe similar theft protection systems for vehicles, such as e.g. shopping trolleys. The system described in the mentioned documents includes a buried cable which defines a predetermined spatial periphery. The system includes furthermore a receiver which is assigned to a wheel of the vehicle. The receiver is designed to receive a predetermined signal and, in response thereto, to move a locking component for locking the wheel from a released state into a locked state or vice versa. A decision as to whether the locking component is displaced into the released or the locked state is effected on the basis of whether the wheel was locked or was freely moveable when approaching the buried cable.
U.S. Pat. No. 5,315,290 describes a mechanism for locking a trolley. The mechanism responds to signals which describe a permissible usage region of the trolley and is contained completely within the wheel of the trolley. The mechanism comprises a printed circuit board with a high frequency receiver, a decoder and a logic driver. The mechanism comprises furthermore a solid body device. If the receiver detects that the trolley has been moved out of the previously described area of use, the solid body device is supplied with energy in order to lock rotation of the wheel and hence to lock the trolley. The mentioned system is based essentially upon detecting a signal strength of a signal emitted by a central antenna. If the receiver loses the signal, then this indicates that the trolley has been moved out of the previously described area of use. If, in contrast, the signal is found again, then this indicates that the trolley is again situated within the previously described area of use.
U.S. Pat. No. 6,125,972 describes a further safety apparatus for a shopping trolley or other vehicles. A braking component is mounted rotatably on an axle of one of the wheels. The braking component is designed to separate the wheel from the ground in an operating position and thus to lock a movement of the vehicle. A locking device locks the braking component in a raised position. The braking component is furthermore released, in response to a signal from a signalling system which extends around a region in which the trolley is intended to be retained. In this case, the braking component rotates into an operating position and thus stops normal movement of the trolley. The system comprises in addition a buried loop antenna which is buried along a periphery of the region to be monitored. A receiver which actuates the braking component comprises a resonator, a high frequency amplifier, a rectifier, a threshold detector and a pulse generator for actuating a solid body. The locking mechanism is deactivated again furthermore by using a special key. For this purpose, a housing has a keyhole.
US 2006/0247847 A1 describes a navigation system which uses a compound navigation method in order to estimate a current position of an object relative to one or more earlier positions. The compound navigation method determines a change in position from the direction and speed of the object during a time interval. The system comprises inter alia also a magnetic field sensor. With the help of two magnetic field sensors, which are disposed for example along an x axis and along a z axis, for example a magnetic direction marker can be detected in addition. Furthermore, for example a marking with a magnetic strip code (bar code) can be evaluated. A two-axis magnetic sensor is designed for example to detect magnetic field components in the direction of movement of the object and perpendicular to the direction of movement of the object. A z-axis sensor can be used for example to detect the beginning and the end of a bit. A y-axis sensor, in contrast, detects whether the bit concerns a “1” or a “0”.
US 2006/0244588 A1 describes a bidirectional communication system for tracking positions and states of vehicles. The vehicle tracking system comprises a wheel which contains a sensor circuit. The sensor circuit is able to detect different conditions, such as e.g. rotation of the wheel, a vibration of the wheel or specific electromagnetic and/or magnetic signals which indicate a specific position of the wheel. The mentioned document describes inter alia the use of magnetic markings or strips in order to produce an additional position tracking mechanism. Each magnetic strip has an unequivocal magnetic pattern which can be detected by a magnetic field sensor which is contained in each wheel. Furthermore, the system includes a signal line for a signal with a very low frequency. A code which is transmitted via this line can correspond unequivocally to the entrance or exit of a shop. Entry of the trolley into the shop can be distinguished from leaving the shop in that a timing, with which the transceiver-receiver of the trolley receives the signal with the very low frequency, is evaluated relative to a timing with which the trolley sees different RSSI levels from access points mounted at the exits.
U.S. Pat. No. 6,161,849 describes a steering wheel locking device for use with shopping trolleys which have at least one articulated wheel. The locking device locks the rotatable wheel in a rotated configuration so that the trolley can move merely along a circular path. Resetting of the locking device can be effected for example by using a portable transmitter. As an alternative thereto, also a mechanical key can be used to reset the locking.
U.S. Pat. No. 5,357,182 describes a further theft prevention system for shopping trolleys. The system aims to brake a vehicle if an attempt is made to move the vehicle out of a defined periphery. At least one wheel of the vehicle is coupled mechanically to an axle which carries a device in order to produce an electrical current in response to a rotation of the axle. The system comprises furthermore a circuit which is designed to short circuit the terminals in response to a predetermined signal in order hence to prevent rotation of the axle and to brake the wheel. The system includes furthermore a device for producing the predetermined signal and a device for local emission of a signal along the periphery. In the mentioned system, two antennae which emit different signals are used. A signal which is emitted from the first antenna effects locking of the wheel. A signal which is emitted from the second antenna effects release of the locking of the wheel.